Patrick Ferris 2026 3 19 https://patrick.sirref.org/weekly-2026-w12/ weekly-2026-w12 /weekly-2026-w12/ A POSIX Shell in OCaml Long time, no weekly. Since the start of this year I have been building a POSIX shell in OCaml called mshMerry Merry is available online now. Patrick Ferris 2026 3 19 MerryEio alongside Morbig Patrick Ferris 2026 3 19 Shells have been around for a long time. In my research on the notion of metashell I wrote about Louis Pouzin originally coining the term: Patrick Ferris 2025 5 19 https://patrick.sirref.org/pouzin-shell/ pouzin-shell /pouzin-shell/ Metashells › Louis Pouzin's "SHELL" I spent some time reading part of the multics design documentation this week. Louis Pouzin coined the term "SHELL" in this document, and I was reminded yet again just how important it is to be a good writer even as a "computer science researcher". For example, this excerpt from the requirements section of the document The previous definitions imply that a command MUST be designed while keeping in mind the user, sitting at his console, wondering about what might be going on, mistyping or forgetting arguments, even if fully aware of the conventions, and possibly interfering with the command by hasty quits, carriage returns, and other temperamental reactions. And then later, when defining the "SHELL". We may envision a common procedure called automatically by the supervisor whenever a user types in some message at his console, at a time when he has and subroutine. The purpose of such a procedure is to create a medium of exchange into which one could activate any procedure, inside of another program if it were called. Hereafter, for simplification, we shall refer to that procedure as the "SHELL". It still surprises how little the undergraduate degree in computer science at Cambridge focuses on writing skills. I built Shelter exploring the idea of a shell-like interface that allowed users to undo shell-like is not enough. Shelter cut many corners to masquerade as a env to understand how a command may have altered the Shelter a "SHELL"! To do that, I needed a solid foundation to build on. msh, the POSIX shell that comes with Merry, is by no means POSIX-complete in terms of features. But it is my daily driver at this point. It includes a pure OCaml rewrite of linenoise readline bruit. If you have docker installed on your machine, you can take it for a spin today: The patrickferris/msh docker image is just for trying it out. It is based on the OCaml 5.3 alpine image. Alternatively, you can build msh from source and have it available in your opam switch. There are many small paper cuts left to patch over, but most of it is porcelain ctrl+left-arrow for moving in bruit of things that you will immediately stumble upon. Patrick Ferris 2026 3 19 What makes Merry different? Nothing. Merry is supposed to be a solid, POSIX-ish base to build on. Unfortunaltely, as it turns out, the subset of features from the POSIX specification that people actually use... is pretty much all of it. Every possible redirection, variable expansion, shell built-in and compound command make some appearance. Not to mention the non-POSIX bits of shell we all take for &> Merry and msh are useable. You will most likely find bugs if you use them. If, when using msh, you find something obscure happening you can enable debug mode either my setting the variable MSH_DEBUG or by invoking msh with -v -v. Patrick Ferris 2026 3 19 What's next? It is soon time to combine Shelter and Merry into the time-travelling, POSIX-ish shell that I have been trying to build since I first started working on Shelter. For this to be successful, I need Merry to be able to pass plenty of tests and right now that involves trying to install plenty of packages using tools like apk and apt. Patrick Ferris 2026 3 19 Shell MRDT What I am particularly interested in reasoning about, is the execution context in Merry. This value, alongside the file-system, constitutes a fairly deep understanding of the state that changes in each step of a shell's evaluation loop. This was, in terms of Shelter, the missing piece for truly building some kind of MRDT across shell sessions. Patrick Ferris 2026 3 19 TIFF in OCaml I picked up from the excellent Outreachy work of Tambe Salome in getting write-support in ocaml-tiff. We are getting closer to the kind of API I envisaged in this latest round of refinement and review. let / = Eio.Path. / let checkerboard ~size = let v = Nx.zeros Nx.uint8 size; size in for row = 0 to size - 1 do for col = 0 to size - 1 do if row + col mod 2 = 0 then Nx.set_item row; col 254 v done done; Nx.to_bigarray v let = Eio_posix.run @@ fun env -> Tiff_eio.with_open_out env#cwd / "example.tiff" @@ fun w -> let tif = Tiff.make checkerboard ~size:256 in Tiff.to_file tif w I have also been extremely pleased to see further external collaborators appearing: Nicolas helping out with metadata maintainence. Virgile adding support for reading multi-image TIFF files. References Louis Pouzin 2013 https://patrick.sirref.org/pouzin-shell-2013/ pouzin-shell-2013 /pouzin-shell-2013/ The Origin of the Shell Reference https://people.csail.mit.edu/saltzer/Multics/Multics-Documents/MDN/MDN-4.pdf Multics Documents Louis Pouzin 2013 Abstract How RUNCOM was created for CTSS and the shell was designed for Multics by Louis Pouzin. Context https://patrick.sirref.org/weeklies-2026/ weeklies-2026 /weeklies-2026/ Patrick Ferris' Weeklies › 2026 Patrick Ferris 2026 3 30 https://patrick.sirref.org/weekly-2026-w13/ weekly-2026-w13 /weekly-2026-w13/ Ppxlib release and Merry updates Patrick Ferris 2026 3 30 https://patrick.sirref.org/ppxlib-release-0-38-0/ ppxlib-release-0-38-0 /ppxlib-release-0-38-0/ Ppxlib Releases › 0.38.0 Nathan and I released ppxlib.0.38.0 last week. Its main feature is full migration support for the upcoming OCaml 5.5 alpha3 of new features landing in OCaml 5.5: modular explicits, external type declarations and arbitrary "local" structure items. As Nathan and I continue to find a plausible maintenance story for ppxlib, Nathan has opened an issue on the OCaml compiler to discuss the idea of adding additional extension points to the language. This comes from the desire to be able to store encoded versions of new OCaml features inside older abstract syntax trees. There are also some nice bug fixes in there too: A potential OOM loop has now been removed. Locations have been restored to long identifiers! Patrick Ferris 2026 3 30 Merry updates number of slightly obscure features that one needs to support. I think this is surprising because most people writing shell scripts write simple shell scripts; scripts that use a much smaller subset of features. In the same breath, those same developers are likely to use something like apt-get install bash Patrick Ferris 2026 3 30 Exec redirects In a shell script, you can use the built-in command exec to replace the current exec vimdifferent mode of operation for exec: If exec is specified with no operands, any redirections associated with the exec command shall be made in the current shell execution environment. One of my litmus tests for Merry is the Debian debootstrap scriptsAnil One thing that it does is the following: err printf "err $1" >&4 exec 4>&1 A lot of detail has been elided for clarity. exec 4>&1 sets up a redirection for the shell's execution environment in which file descriptor 4 is now an alias for standard output. So, writing to 4 standard output to 4 i.e. >&4 For this to work, shell's must dup2 the relevant file descriptors which have the following condition: If the file descriptor newfd was previously open, it is closed This is all very well, unless your program has an important file already open that happens to have file descriptor 4. The Eio Linux and POSIX backends suffer from this problem. Both make use of a file-based synchronisation mechanism for waking up the eventloop should another domain push a completion to the scheduler's run queue. On Linux this is via an eventfd and in POSIX, a pipe. This dup2 will close the eventfd assert falseEio and moving the eventfd file descriptors to higher values, though I have opened a PR to make this more configurable. This bug is quite easy to write up... it was not so easy to find! As a random example, consider the debconf/confmodule script which offers very little room for a buggy implementation! Patrick Ferris 2026 3 30 Outreachy The presentations from the demonstrations for this round of Outreachy are now online! Patrick Ferris 2026 2 9 https://patrick.sirref.org/outreachy-ocaml-tiff/ outreachy-ocaml-tiff /outreachy-ocaml-tiff/ Write support in OCaml TIFF library I am mentoring Tambe Salome during the December 2025 Outreachy round to add support for writing TIFF files in the ocaml-tiff library. You can now see the video of the demonstration day presentation: Patrick Ferris 2026 3 19 https://patrick.sirref.org/weekly-2026-w12/ weekly-2026-w12 /weekly-2026-w12/ A POSIX Shell in OCaml Long time, no weekly. Since the start of this year I have been building a POSIX shell in OCaml called mshMerry Merry is available online now. Patrick Ferris 2026 3 19 MerryEio alongside Morbig Patrick Ferris 2026 3 19 Shells have been around for a long time. In my research on the notion of metashell I wrote about Louis Pouzin originally coining the term: Patrick Ferris 2025 5 19 https://patrick.sirref.org/pouzin-shell/ pouzin-shell /pouzin-shell/ Metashells › Louis Pouzin's "SHELL" I spent some time reading part of the multics design documentation this week. Louis Pouzin coined the term "SHELL" in this document, and I was reminded yet again just how important it is to be a good writer even as a "computer science researcher". For example, this excerpt from the requirements section of the document The previous definitions imply that a command MUST be designed while keeping in mind the user, sitting at his console, wondering about what might be going on, mistyping or forgetting arguments, even if fully aware of the conventions, and possibly interfering with the command by hasty quits, carriage returns, and other temperamental reactions. And then later, when defining the "SHELL". We may envision a common procedure called automatically by the supervisor whenever a user types in some message at his console, at a time when he has and subroutine. The purpose of such a procedure is to create a medium of exchange into which one could activate any procedure, inside of another program if it were called. Hereafter, for simplification, we shall refer to that procedure as the "SHELL". It still surprises how little the undergraduate degree in computer science at Cambridge focuses on writing skills. I built Shelter exploring the idea of a shell-like interface that allowed users to undo shell-like is not enough. Shelter cut many corners to masquerade as a env to understand how a command may have altered the Shelter a "SHELL"! To do that, I needed a solid foundation to build on. msh, the POSIX shell that comes with Merry, is by no means POSIX-complete in terms of features. But it is my daily driver at this point. It includes a pure OCaml rewrite of linenoise readline bruit. If you have docker installed on your machine, you can take it for a spin today: The patrickferris/msh docker image is just for trying it out. It is based on the OCaml 5.3 alpine image. Alternatively, you can build msh from source and have it available in your opam switch. There are many small paper cuts left to patch over, but most of it is porcelain ctrl+left-arrow for moving in bruit of things that you will immediately stumble upon. Patrick Ferris 2026 3 19 What makes Merry different? Nothing. Merry is supposed to be a solid, POSIX-ish base to build on. Unfortunaltely, as it turns out, the subset of features from the POSIX specification that people actually use... is pretty much all of it. Every possible redirection, variable expansion, shell built-in and compound command make some appearance. Not to mention the non-POSIX bits of shell we all take for &> Merry and msh are useable. You will most likely find bugs if you use them. If, when using msh, you find something obscure happening you can enable debug mode either my setting the variable MSH_DEBUG or by invoking msh with -v -v. Patrick Ferris 2026 3 19 What's next? It is soon time to combine Shelter and Merry into the time-travelling, POSIX-ish shell that I have been trying to build since I first started working on Shelter. For this to be successful, I need Merry to be able to pass plenty of tests and right now that involves trying to install plenty of packages using tools like apk and apt. Patrick Ferris 2026 3 19 Shell MRDT What I am particularly interested in reasoning about, is the execution context in Merry. This value, alongside the file-system, constitutes a fairly deep understanding of the state that changes in each step of a shell's evaluation loop. This was, in terms of Shelter, the missing piece for truly building some kind of MRDT across shell sessions. Patrick Ferris 2026 3 19 TIFF in OCaml I picked up from the excellent Outreachy work of Tambe Salome in getting write-support in ocaml-tiff. We are getting closer to the kind of API I envisaged in this latest round of refinement and review. let / = Eio.Path. / let checkerboard ~size = let v = Nx.zeros Nx.uint8 size; size in for row = 0 to size - 1 do for col = 0 to size - 1 do if row + col mod 2 = 0 then Nx.set_item row; col 254 v done done; Nx.to_bigarray v let = Eio_posix.run @@ fun env -> Tiff_eio.with_open_out env#cwd / "example.tiff" @@ fun w -> let tif = Tiff.make checkerboard ~size:256 in Tiff.to_file tif w I have also been extremely pleased to see further external collaborators appearing: Nicolas helping out with metadata maintainence. Virgile adding support for reading multi-image TIFF files. Backlinks Related Patrick Ferris 2026 2 9 https://patrick.sirref.org/merry/ merry /merry/ Merry https://tangled.org/patrick.sirref.org/merry Merry is a library for building POSIX-ish shells in OCaml. It comes with a Eio called msh. Supporting the full POSIX specification is no easy feat, however, msh already supports plenty of the more useful bits of the spec. https://patrick.sirref.org/eio/ eio /eio/ Eio https://github.com/ocaml-multicore/eio Eio provides an effects-based direct-style IO stack for OCaml 5 Patrick Ferris https://patrick.sirref.org/mrdts/ mrdts /mrdts/ Mergeable Replicated Data Type Implementation Definition A for a data type is a tuple where: is the set of all possible states at a branch, is the initial state, implements every data type operation, implements the three-way merge strategy. Definition 2.1 from "Certified Mergeable Replicated Data Types". https://patrick.sirref.org/outreachy/ outreachy /outreachy/ Outreachy Project https://www.outreachy.org/ Outreachy provides internships in open source and open science. Outreachy provides internships to people subject to systemic bias and impacted by underrepresentation in the technical industry where they are living. I am one of the coordinators for the OCaml community for Outreachy. I have also mentored multiple three-month internships for the community. Patrick Ferris 2026 2 9 https://patrick.sirref.org/outreachy-ocaml-tiff/ outreachy-ocaml-tiff /outreachy-ocaml-tiff/ Write support in OCaml TIFF library I am mentoring Tambe Salome during the December 2025 Outreachy round to add support for writing TIFF files in the ocaml-tiff library. You can now see the video of the demonstration day presentation: 2022 12 1 https://patrick.sirref.org/topojson-0001/ topojson-0001 /topojson-0001/ Implement a non-blocking, streaming codec for TopoJSON https://github.com/geocaml/ocaml-geojson GeoJSON is a JSON standardised format for handling simple geographical data. One common use of GeoJSON is to add map overlays such as with the popular JavaScript library leaflet. One issue with GeoJSON is redundant data, for example when two polygons share boundaries ideally only one would contain the full data about the boundary, however, with GeoJSON the data is duplicated. TopoJSON is an extension to GeoJSON to encode topology. This allows for redundant data to be removed and file sizes to be greatly reduced. This is often very desirable especially when working with data in the browser. In a previous Outreachy internship a new OCaml library was implemented to provide an OCaml library for TopoJSON, this project will build on this adding more functionality to the library and providing a non-blocking, streaming codec version similar to the geojsone library. 2022 6 1 https://patrick.sirref.org/topojson-0002/ topojson-0002 /topojson-0002/ Extend OCaml's GeoJSON library to support TopoJSON https://github.com/geocaml/ocaml-geojson GeoJSON is a JSON standardised format for handling simple geographical data. One common use of GeoJSON is to add map overlays such as with the popular JavaScript library leaflet. One issue with GeoJSON is redundant data, for example when two polygons share boundaries ideally only one would contain the full data about the boundary, however, with GeoJSON the data is duplicated. TopoJSON is an extension to GeoJSON to encode topology. This allows for redundant data to be removed and file sizes to be greatly reduced. This is often very desirable especially when working with data in the browser. In a previous Outreachy internship a new OCaml library was implemented to provide an OCaml library for TopoJSON, this project will build on this adding more functionality to the library and providing a non-blocking, streaming codec version similar to the geojsone library. 2021 6 1 https://patrick.sirref.org/ocamlorg-0001/ ocamlorg-0001 /ocamlorg-0001/ Improve the ocaml.org Website https://ocaml.org OCaml.org is the main website for OCaml, a functional, typed, high-level programming language. This project revolves around improving the website on multiple different fronts including: layout, accessibility and content. Because a lot of the content of the website is about OCaml and the website itself is written in OCaml, this would be a great opportunity for interns to learn the language and functional programming concepts. https://patrick.sirref.org/shelter/ shelter /shelter/ Shelter https://tangled.sh/@patrick.sirref.org/shelter A shim between a user and the shell that provides greater reproducibility and insight into what your code is doing! Give me Shelter! Shelter combines the caching and reproducibility of Dockerfiles with the ease of interaction of the shell. I'd be interested to use Ryan's package management work as a means to specify the base environment users want. Here is an example of running Shelter and making use of the branchable sessions. shelter> echo hello > hello.txt shelter[main#bbd691a] : { mode: rw }> @ session exp-1 shelter[exp-1#bbd691a] : { mode: rw }> echo world >> hello.txt shelter[exp-1#dd5bab8] : { mode: rw }> @ session main shelter[main#bbd691a] : { mode: rw }> echo "to the" >> hello.txt shelter[main#b9abef4] : { mode: rw }> cat hello.txt hello to the shelter[main#d1c1728] : { mode: rw }> @ session exp-1 shelter[exp-1#dd5bab8] : { mode: rw }> @ replay main shelter[exp-1#df1b4e1] : { mode: rw }> cat hello.txt hello to the world shelter[exp-1#67162b7] : { mode: rw }> The @ character allows the user to interact directly with Shelter's build-in operators. @ session exp-1 creates a new session as there is no exp-1 session. git checkout -b exp-1 From there we make changes and switch between branches until we perform a replay. A replay is a rebase without any merge conflict checks, it simply finds the least common ancestorexp-1main Under the hood Shelter is a light-weight, reimplementation of obuilder. ZFSVoid Between the low-level store and the interactive shell parts, there is an Irmin store. https://patrick.sirref.org/ocaml-tiff/ ocaml-tiff /ocaml-tiff/ ocaml-tiff https://github.com/geocaml/ocaml-tiff Part of the geocaml suite of geospatial libraries for OCaml. Contributions